Capacitive touch sensors are used as a user interface to electronic equipment, e.g., computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch sensors provide soft key functionality. In other applications, transparent touch sensors overlay a display to allow the user to interact, via touch, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch sensors are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, e.g., a user's finger tip, causes the capacitance thereof to change.
One way to detect changes in capacitance on a touch sensor utilizes what is known in the art as a relaxation oscillator. The relaxation oscillator drives an oscillating electric signal onto the conductive elements (e.g., sensors) of the touch sensor while a sensing circuit monitors the frequency of oscillation of the driven elements. When an object contacts the touch screen, the resulting change of capacitance causes the frequency of oscillation of the driven elements to change, indicating a touched condition.
One problem associated with using a relaxation oscillator-based capacitive touch sensor is that conducted (common mode) noise present on the power supply connections of a capacitive touch sensor can cause interference, false, triggering, and/or out of range values due to the noise overdriving the capacitive touch relaxation oscillator. When this occurs, frequency shifts may be exaggerated, sensitivity, may be significantly increased, and noise at the unpressed frequency may not be detectable as a frequency shift (e.g., blind spots). Current relaxation oscillator-based capacitive touch sensor systems employ measures to either reduce the conducted noise (e.g., filtering) or limit the system's susceptibility to the conductive noise (e.g., overdriving). However, these approaches have drawbacks. For example, they may require additional or more expensive circuit components.